A cask used, for example, when transporting a radioactive material such as a spent nuclear fuel, must have a structure that efficiently dissipates heat generated by the radioactive material stored inside the cask to the outside, and that also shields gamma rays and neutrons emitted from the radioactive material so that they do not escape to the outside.
According to Patent Reference 1, for example, there is disclosed a prior art cask for transporting radioactive material that has a lead layer serving as a gamma ray shielding material interposed between a stainless steel inner shell and a steel intermediate shell disposed on the outer side of the inner shell. The cask according to Patent Reference 1 is also filled with a silicone rubber that serves as a neutron shield and is interposed between the intermediate shell and a steel outer shell disposed on the outer side of the intermediate shell.
Typically, such a multilayer lead cask is a cylinder with a three-layer structure (in the following, the structure on the innermost side is referred to as an “inner shell”; the structure on the outermost side is referred to as an “outer shell”; and the structure between the “inner shell” and the “outer shell” is referred to as an “intermediate shell”). A lead layer with outstanding gamma ray shielding properties is formed between the metallic inner shell and the metallic intermediate shell by pouring molten lead between the inner shell and the intermediate shell, and allowing the lead to solidify. This ensures a shielding capability against gamma rays, while forming an enclosure that is as thin as possible. The prior art technology according to Patent Reference 1 is an example of a method of forming a lead layer in a casting process involving a pouring of molten lead.
However, when a lead layer is formed between the steel shells of a two-layer structure, voids readily form at the boundary between the inner shell and the poured lead, or at the boundary between the intermediate shell and the poured lead, simply by pouring molten lead between the inner shell and the intermediate shell. If gases are present inside these voids, there are some cases in which the state is nearly that of a virtual vacuum, but in any case, when such voids (referred to below as “void layers,” irrespective of the presence or absence of gases) exist, the heat-dissipating effect of the cask is significantly reduced. For this reason, if the cask is used without removing such void layers, the internal temperature of the cask exceeds a hypothetically allowable temperature, resulting in a hazardous state.
Patent Reference 2 was proposed to prevent the formation of such void layers, by employing what is generally referred to as a “homogenization treatment” before pouring the molten lead between the inner shell and the intermediate shell, so as to enhance adhesion between the lead layer and the steel shells.
In the past, homogenization treatment was employed by heating lead with a burner to melt it so as to create an alloy layer, while causing the lead layer to adhere more closely to the alloy layer and successively increase the thickness. However, the object of the manufacturing method disclosed in Patent Reference 2 was to improve the adhesion obtained in homogenization treatment by forming a vitrifiable lead-tin based thin-film. Specifically, the homogenization treatment according to Patent Reference 2 was implemented with the following sequence of steps: (1) A washing treatment step in which adhering matter, greasy components, and the like are removed from the external surface of the inner shell by degreasing, to produce a clean state; (2) A solvent application step in which the steel sheet surface is heated with a burner to a temperature on the order of 230-270° C., and after the surface reaches a specified temperature, a flux which is a solvent that improves wettability is applied; (3) A vitrifiable material application step in which the vitrifiable lead-tin based material is uniformly applied to the surface by dissolving it and dropping it onto the surface immediately after solvent application; and (4) A thin-film formation step in which the system is cooled for a while after solvent application, the inner surface (the side that holds the radioactive material) of the inner shell is reheated with a burner, the temperature is raised to 180-250° C., and the floating vitrifiable material is wiped off with a heat-resistant cloth, forming a thin-film of vitrifiable material on the exterior surface of the inner shell.
However, homogenization treatment that requires steps such as (1) to (4) has a problem in that because it is accomplished almost entirely by manual labor performed by highly experienced and highly skilled workers, it is very inefficient, it takes an extended period of time to manufacture the cask, and the manufacturing cost is high.
In addition, due to the fact that it is not possible to always prevent the formation of void layers when the above-described homogenization treatment is employed, there is a need to inspect the cask after it is manufactured to see whether or not there are void layers, and the inspection process itself takes a lot of work.
The present invention was devised with consideration given to the above-described problem of the formation of void layers at the boundary between the inner shell and the poured lead, or at the boundary between the intermediate shell and the poured lead. The object of the present invention is to provide a containment cask for radioactive material that makes it possible to shorten the manufacturing time and to reduce the manufacturing cost by completely eliminating homogenization treatment, or, even if homogenization treatment is employed, to reduce the scope of its use by half.